Mechanical filter



July 29, 1952 l.. l.. BURNS, JR., ETAL A 2,605,354

' MECHANICAL FILTER Filed March 22, 1949 k k a I Q I l I r Il s Jl Ik`-, .v/ u

FREQUENCY l FREQUENCY Fg. 2 Fg. 4

f l 'JQ l 70 /V//V CO/L 7 ATTORNEY Patented July 29, 1.952

, ,UNITED A STATI-:s Pli'rlalvrA ori-lcs MEHANICAL FILTER Leslie L. Burns, Jr., and .Walter van B. Roberts,

Princeton, N. J., assi'gnors to Radio Corporation ofAmverica, a corporation of Delaware Application March 22, 1949, Serial No. 82,834

v12 Claims. 1

, 2 resonant body acts to sharply reduce the transmission through the filterl at the resonant Vfrequency of such body. By the use of such a body,

, very high attenuation maybe lproduced at a predetermined frequency which may be located at any desired pointoutside the pass band of the filter. In this way, rejection of a particular frequency may be produced, and the arrangement of this invention may be'termed a rejecton frequency-response characteristic of a mechani- 1 cal-typev band pass filter. j

A' still further object is to provide a simple, in'- expensive yet effective means for effecting high attenuation in a band pass filter ata, frequency usually outside the pass band,but which may be inside such band if desired. y 1

,The foregoing and other objects of the invention will be 'best vunderstood' from the following description Yof an exemplification thereof, reference being' Iliad to the accompanying drawing, wherein: n

Fig. l is a schematic representation of one embodiment of this invention; y

' Fig. 2 is a set of curves illustrating certain characteristics of operationvof Fig. 1;

Fig. 3 is a schematic representation of a modification; and

Fig. 4 is a set of curves illustrating certain characteristics of operation of Fig. 3.

A filter is usually used between amplifier tubes or other electronic amplifier devices. For greatest stage gain, the filter should be matched to the input and output impedances of such tubes; This invention is concerned with electromechanical filters of the magnetostrictively-driven type. The

drive and pickup 'coils' on the ends of such al filter are likely to be of rather low impedance and the power factor high, because of the electromechanical coupling to the filter and'because of eddy currentlosses in the end tanks of the filter. A tunable impedance matching device should, therefore, be used.

For tuning the filter drive or pickup coils, or for matching the impedance of such coils to that ofthe tubes with which they are used, separate resonant impedance matching networks, each consisting ofnan auxiliary coil and a, condenser, are used, such networks being connectedto the corresponding drive or pickup coil. In accordance with this'invention', amechanically resonant body, tuned to a predetermined resonant frje-` quency, is used'asa tuning core for the auxiliary coil of the impedance matching network; This Now referring to Fig.. 1', numeral l indicates generally an electromechanical filter. 'Ihis filter may be, for example, of the multiplesection ballcoupled type, as more particularly described in our copending application, Serial No. 84,372, led

March l30, 1949. In an example given as a specific embodiment, the mechanical filter I consisted of four ball-coupled sections, the 15-6 Ydiameter steel balls 2, 3, 4 and 5 beingsoldered on a length 6 of .004 wall nickel tubing of .06" outer diameter at intervals of .92 betweencenters of the balls, the end sections being .46 long, Thus, each of the four sections of the filter can be Aconsidered to be a ball centrally mounted on a length of tubing which is .92 long.

Although the invention has been illustrated and described above as being utilized in connection with a multiple-sectionball-coupled filter, and although specific dimensional values. have been previously given, it is desired to be made clear at this juncture that the invention is equally applicable to numerous ,typesof magnetostrictive electromechanical band passY filters, such as any of the types disclosed inthe aforementioned vcopending application. vIniother words, the invention is applicable to various types'fof frequency selective filters. In fact, Ythe invention Vis not limited to magnetcstrictivelydriven filters, nor even to electromechanical filters;l the invention can also be used in conjunction withelectrical band pass filters of any type, the onlyrequirements being that van impedance matchingnet- Work be provided between lthe band pass lter and another impedance, and vthat AanA auxiliary coil or inductance costnitute part vof this matching network.

This statementV Will becomemore clearly apaprent as the descriptionproceeds.Kl

yTo proceed with the description of the specific embodiment given above as an example, the particular lter I described above passed a band from 99 to 101.3 kilocycles, and at-98.64kilocycles the outputQWas .downfto 1/15 of the'output at 99 kilocycles. This, avery rapid-cutfoi by ordinary standards, but the iilterwas intended vtobe used vfor single side".l band selection,-where an extremely'rapid cut-off is needed.

have rather high input and output impedances,

or between other devices or elements which have' high impedances. v v

For greatest stage gain, or in `other words, for the maximum efficiency of transfer vof energy, theV coils 'l and 8 should be matched to the impedances of the devices between which such coils are connected. For this purpose, tunable impedance matching networks are provided between the filter coils and the driving and pickup tube stages, each of these impedance matching networks consisting of an L-section LC network, the YL of each such network being tuned by a ferromagnetic tuning core. Y Y

VA variable condenser 9, which is the C of the LC network, is connected across the plate-cathode or output circuit (of avdriver amplifier tube I0, the cathode of this tube being grounded as shown. The lower grounded plate of condenser 9 is connected directly to one end of driving coil 7, while the upper or high potential plate of such condenser is connected to Vone'end of an lauxiliary coil Il which is mounted on a suitable coil form I2 made of insulating material. The auxiliary coil II serves as the L of the LC impedance matching network, which is located between the band p'asspfilter I and tube Iil and which matches the high output impedance of such tube to the'filter drive coil 'I. The other end of coil II is connected to that end ofv driving coil'I opposite to the grounded end thereof. Thus, the two coils 'I and I'I are connected in series across the driving source ID output, and' condenser 9 is also connected across said output.

According tothe present invention, a ferrite resonator rod I3, tunedV to 98.4 kilocycles, is placed inside auxiliary coil I I, to serve as a tuning core to tune such coil. In vother words, rod I3 is made of a ferromagnetic material and helps to provide the appropriate value of `inductance for coil II, they appropriate value L of inductance `being that necessary for giving the desired impedance transformation or match between driving coil 'I and driving tube I0.

The tubing element '6 is made of magnetostrictive material, and magnetostrictive drive and pickup of said element is eectedV through coils 'I and 8, as disclosed in the aforementioned copending application. To provide longitudinal polarizing magnetic flux through element 6, magnets I4 and I5, poled as indicated, are provided adjacent the opposite ends of element E. Of course, if torsional drive of the electromechanical lter were desired, the magnets would be arranged somewhat differently with respect to element 6, as described in said copending application. y

Resonator rod I3, being made preferably of a ferrite material, is also magnetostrictive. Magnet I4 is of such a size and is placed in such a position that it provides polarizing flux for core or rod I3. Again, although the rod resonator I3 has been described above as being of coil II,` as previously described, andsaid coil is Y therefore coupled to suchy resonator to serve as the driving coil therefor. Rod I3 is mounted in Vcoil II in such a way that it is free to mechanalternating voltagein coil II is near its resonant frequency of 98.4 kilocycles.

At the resonant frequency of the resonator I3, the` effective resistance or impedance of its coil II clearly increasesV or '.becomesvery "high, due

to the vibrationof` such resonator. The apparent impedance of coil II ifs greatly' alteredat the resonant frequency 'of rod'l, rthus throwing the resonant impedance matching network 9, II so far out of adjustment as to provide a large impedance mismatch' of the coil 'I to the tube ID at this frequency; 'greatly impairing the efciency of energy transfer to driving coiljI at said frequency and also greatly impairing the efficiency of operation of thei'filter I at and near the rod resonant frequency. In effect, then, the arrangement of this invention, including rod I3 and coil II, absorbs energy Vat the rod resonant frequencyv and substantially prevents vit from reaching and being applied toA drivingcoil .'I of the filter I, due to the large. impedance mismatch provided at and near such rod resonant frequency. Considering theA arrangement fof Fig. 1 as a whole, the rod I3Y with its coilv II .may

be termed a"rejector,fsince Vit vin effect acts to reject energy of a predetermined frequency for to substantially prevent energy of such frequency from reaching filter I.'

The arrangement of the particular embodiment being described operatedfto reduce 'the nlter output to about 1/300 of maximum, at a frequency of 98.6 kilocycles, and t'o an imrneasurably small valuefor all frequencies below 98.6.

It will be seen, from the'foregoing, that the rod resonator I 3 provides a single means for tuning the impedance matching network '9, `II between a band pass lterfl and a tube I0, and also for` improving 'the` attenuation outside the pass band'of the filter `I byjintr'oducin'g a frequency of very high attenuation at any desired location outside 'the pass band.

Fig. 2 illustrates the very desirable results ob-` tained'by the use of Athis invention. lnthis iigur'e, the solid line lcur'verepresents thefrequencyoutput characteristic` ofan electromechanical band pass filter such as filter I, without the re- `iector of this invention, while the dotted-line curve represents the characteristicsl ofthe 'same nlter utilizing thev invention vdescribed above,

that is, using a rejector inthe input'iorfd-riving matching coil II. By comparing 'the solidline' steepened, and that these desirable ends have,

been'effectuated -by a relativelysimple' and inexpensive means. In particular, from the valves given hereinabove it may be seen that without the rejector of this invention, at 98.6 kilocycles the filter output was down to only 1A5 of the maximum-output at 99 kilocycles, while with the rejector, at 98.6 kilocycles the filter output was down to 1,500 of the maximum output at 99 kilocycles.

It might be thought that still sharper cutoff could be obtained by using'a rejector resonator frequency still closer lto the lower edge 'of the band, for'instance a frequency of 98.9 kilocycles,

it being recalled that the pass band of the par.-

ticular filter being described extendsfrom 99 to 101.3 kilocycles. However, it must be remembered that the resonator I3 is effective over Vanite narrow band, and it would be undesirable to locate it'so clos-e to the pass band as to affect the filter output within such pass band.- On the other hand, if the magnetostrictive activity of the rod I3 is too much reduced, as by weakening the magnetic field on it, it does not have sufficient rejeetorv effect even at its exact resonant frequency. A compromise of the sort described above has been found to give very satisfactory results.V

The compromise is most easily effected by a cut and try process, but some pr-edetermination may be made from the following considerations. The mechanical Q of a ferrite rod I3 in a coil I I as shown in Fig. l isl on the order of 1000. Hence, said rodv will be appreciably excited over a range of frequencies on the order of T of 1% wide. To avoid interference with the response inside the band, the ferrite frequency should therefore be kept outside the band by considerably more than 1/20 of 1% of the operating frequency. In the example given above, the ferrite was tuned a little more than 1K2 of 1% below the band edge. Perhaps it could have been a little closer, but as the result was quite satisfactory it wasnot considered desirable to make it any closer, as any variation in frequency of eitherv the filter lI or the ferrite I3 (due, for example, to temperature change or to change in the magnetic field on'the ferrite) might cause interference with the pass band.

f, Where a second v'rejection point is desired, a second ferrite rod resonator :may be placed in the output or pickup voltage stepup circuit, as shown in Fig. 1. Here, the high-impedance grid or input circuit of the following amplifier stage isconnected to the low impedance pickup coil 8 through an L-section LC resonant impedance matching network which has circuit connections similarto the impedance matching network on the input side of the filter, previously described, but the impedance values of which may be different.v A variable condenser I6 is connected across the grid-cathode or input circuit of a pickup amplier tube I1, the cathode of this tuber` being grounded as shown. The lower grounded plate of condenser I6 is connected directly to one end of pickup coil 8, while the upper or high potential plate such condenser is connected to one end of an auxiliary coil I8 which is mounted on a suitable coil form I9 made of insulating material. The auxiliary coil I8 serves as the L ofthe LC'impedance matching network, the'C of which is provided by condenser I6, this network'being located between the band pass filter I and tube I'I and matching the high input impedance of such tube to the filter pickup coil 8.- ,'Ihe'other endof coil I8 is conwhich' may be atthe upper edge of the pass band of filtery I or which may be at any other predetermined frequency, iis placed inside auxiliary coil I9. Magnet I5 is,of such a sizeand is placedin such a position that 4it provides polarizing flux for coreorr rod 20., Coil'I8,-is lcoupled to rod resonatory 2B and serves as the magnetostrictivc drivingvcoil therefor. -L'

Rejectorgzandcoil- =I8 act together, in the same manner as d o rejector I3 `and coil I I,` to provide a secondfrequency Arejection point at and near the resonant frequency of rod 20.

lThus, rod resonator 20 provides a single means for tuning the impedance matching network I6, I8 between a band pass filter I and a tube I'I, and also for improving the attenuation outside the'pass bandvof the yfilter I by introducing a second frequency of very high attenuationV at any. desired location outside the pass band.

In most cases, it is desirable to provide'a grounded electrostatic and electromagnetic shield 2I between the driving coil'l and the pickup coil 8. A :shielding structure very suitable for this purpose is disclosed inthe copending Roberts application, Serial No, 76,586, filed February 15,

It has been found.l thatnthe simple rod resonators such asl3-and 20 described above, a1- though `they operate-overa `finite width band, provide rejection lcharacteristics so-very sharp that the width of the rejection band is almost nil. For some applications ofthe invention, this may be an undesirable condition. Itis therefore within the scopeY of this invention to'replace the simple rod resonators of Fig. l'by band pass filters for rejection purposes. Fig. 3 illustrates such a modification.`

Referring now to Fig. 3wherein the illustration is simplified and wherein elements the same as those of Fig. 1 are denoted by the same reference numerals elements II and 9 are the respective components` of the L-section LC resonant impedance matching network on the input side for connecting vthe driving coil of the main filter (not shown) to the output impedance of tube I0, while elements I8 Yand I6 are the respective components` of the L-section LC resonant impedance matching network on the output side for connecting lthe pickup coil of the main filter to the input impedance of tube I'I.

Inductively vcoupled. toauxiliary coil II of the input impedance matching network, in such a way thatsaid coil willfserveas the driving coil thereof, is one tank end offa' band'pass filter denoted generally by 22. vFilter 22 is in general somewhat similarfto the y.filter I of Fig. 1, in that theforrner is also of the multiple-section `ball-"coupled type. However; filter 22 consists of two ball-coupled'lter sections, the twoballs 23 and 24 being soldered'on a length 25 Yof nickel tubing. Also, the filter 22,insteadl ofhaving a pickup coil coupled to the end tank vthereof opposite from the input coil II. has a piece 26 of lossy material, such as the lossy plastic material known asViscoloid, firmly -attached tothe end of tubing length 25g-'opposite to vthat endl to which driving coil II i vcdupled. By this conaccsc 7 Y struction, mechanical vibrations of the electromechanical band pass filter122,in the band passed by said filter, are; converted into heat in member 28, rather than being taken oif for utiliza-v tion by means of a pickup coil as in main filter I.

Similarly, inductively coupled to auxiliary coil I8 of the output impedance matching network, in Ysuch a way -that said coil will serve as the driving coil thereof, is one tank` end of a band pass filter denoted generally by 21". Filter 21 is similar to filter 22, and consists of two ballcoupled filter' sections, the two balls` 28 and 28 being soldered on aV length 3D of nickel tubing. A piece 3l of Viscoloid or other lossy material is firmly attached', asby cementing. to the end of tubing length 30 opposite to that end to which driving coil i8 is coupled. Mechanical vibrations vof--tlfiefband pass fiter 21, in the band passed 'by suol-1V iiItenareA converted into heat in member 3|, rather-than being taken off for utilizationby meansof a pickup coil' as in the main filter.` Y

The operation'of the Fig. 3 embodiment ls very similar to that of the Fig. l embodiment previouslyV described, the main difference being that in Fig. 3 each of the rrejector filters 22 and 21 is in effect resonant over a band, as more fully described in our aforementioned joint ap plication, rather than at substantially only a singlefrequency, as in Fig. vl. In other words, throughout the pass band of each of the rejector lters 22and 21, the impedance of its coupled coil, Ill or I8 as the case `may be. becomes resistive. thus producing an impedance mismatch in the corresponding impedance matching network and a consequentA absorption of energy in such bandsthus-in effect preventing energy of frequencies lying with-inthese bands from being applied tothe main'fi'lter.- Thus, rejector action is produced throughout the pass bands of lters 22 and 21. l i A idg,V iL-illustratf-:s a frequency-output characteristic such as might be obtained with the arrangement of Fig. 3. The solid-line curve represents the characteristic of the main filter-without the use of rejectors, while the dotted-line'curve'- represents the results which might be expected from the use of the Fig. 3 embodiment, that' is, with two band pass filters asV rejectors, these reectors' 22 andy 2-1 havingtheir pass bands at oppositey edges of the pass bandofthe main filter. The steepening of the sides of the main band pass lter characteristic byJ the use of rejector band pass filters should becomeV apparent from an examination o-ffthe dbttedl'ine characteristic of Fig. 4.

It shouldY bev noted, from the above detailed description, that inrFig. 31, aswell as in Fig. 1-, one and the sameV magnet may provide magnetic elds both for the rejector resonator and for the end or tank. element of the mainv filter. Thus, they present inventionrequires no extra parts at all, merely the grinding to. a desired frequency of the corealready useful for tuning thexcoil of. the impedance. matching network; If .used in combination. with an' electrical lter, which use iswithinthe scope ofv this invention,V the'magnet; for polarizing the rejector resonator would be an extra element. A

What `we claim to be., our invention-is as follows :l n

1. Inv combination, a frequency selective mechanical filter ,having electromechanical conversion means coupled-theretm an impedance matching networkVv connecting said Vconversion means to another impedance, said network in- Y 8 cluding an inductance, and. a magnetostrictive resonator coupled to said inductance to be mechanically driven by alternating voltages appearing therein, thereby altering the effective impedance of said inductance and the impedance y match between said conversion means and said other impedance within a range of frequencies about a frequency predetermined by the physical constants of said resonator.

2. In combination, a frequency selective mechanical nlter having electromechanical conversion means coupled thereto, an impedance matching network connecting said conversion means to another impedance, said network including an inductance, and a magnetostrictively-f driven resonant band pass filter coupled to said inductance to be mechanically driven by alternating voltages appearing therein, thereby altering the effective impedance ofsaid inductance and the impedance match between said conversion means and said other impedance over a band of frequencies predetermined by the physical constante of said band pass filter.

3. In combination, an electromechanical band pass lter having electromechanical conversion means coupled thereto, an impedance matching network connecting said conversion means to another impedance, said network including an inductance, and a magnetostrictive resonator coupled to said inductance to be mechanically vibrated by alternating voltagesl appearing therein, thereby altering the effective impedance of said inductance and the impedance match between said conversion means and said other impedance within a range of frequencies about a frequency predetermined by the physical constants of said resonator, said frequency range having a loca-I tion in the frequency spectrum outside the pass band of said filter but near one edge thereof.

fi. In combination, an electromechanical band pass lter having electromechanical conversion means coupled thereto, an impedance matching network connectingsaid conversion means to another impedance, said network including an inductance, and a sharply resonantr magnetostrictive resonator coupled' tosaid inductance to be mechanically vibrated by alternating voltages appearing therein, thereby altering the effective impedance of said inductance and the impedance match between said conversion means and said other impedancewithin a narrow'range of frequencies. about a frequency predetermined by the physical constants of said resonator, said fres quency range having a location in the frequency spectrum outside thepass band of said lter but near one edge thereof.

5. In combination, anelectromechanicalband pass filter having electromechanical conversion meanscoupled thereto, an impedance'rnatching network connecting said conversion means to another iinpedance, said network including an inductance, and .a magnetostrictively-driven resonantbandpass filter coupled to'saidY ind'uctance f to. beV mechanically vibrated by alternating voltages appearing therein, thereby altering the effective impedanceA of said inductance and the impedanceinat'ch between said conversion means and said. other impedance over a band of frequencies predetermined by the physical constants of saidlsecond-named filter, said frequency band having a location in the frequency spectrum outside the pass handrofl. said first-named'fil'ter but near. one edge. thereof.

.6. In combination, a. frequency selective filter a separate impedance matching network coupled to each of said connections for connecting said lter to another impedance, said networks each including an inductance, and a separate magnetostrictive resonator coupled to each of said inductances to be mechanically driven by alternating voltages appearing in its corresponding inductance.

7. In combination, a frequency selective mechanical filter having electromechanical conversion means coupled thereto, an L-section resonant LC impedance matching network connecting said conversion means to another impedance, and a magnetostrictive resonator coupled to the inductance of said network to be mechanically driven by alternating voltages appearing therein.

8. In combination, a magnetostrictively-driven electromechanical lter having driving and pickup coils coupled thereto, an impedance matching network coupled to at least one of said coils for connecting such coil to another impedance, said network including an inductance, and a magnetostrictive resonator coupled to said inductance to be mechanically driven by alternating voltages appearing therein.

9. In combination, a magnetostrictively-driven electromechanical lter having driving and pickup coils coupled thereto, a separate impedance matching network coupled to each of said coils for connecting such coils to corresponding impedances, said networks each including an inductance, and a separate magnetostrictive resonator coupled to each of said inductances to be niechanically driven by alternating voltages appearing in its corresponding inductance.

10. In combination, a magnetostrictively-driven electromechanical filter having driving and pickup coils coupled thereto, a separate impedance matching network coupled to each of said coils for connecting such coils to corresponding impedances, said networks each including an inductance, and a separate sharply resonant magnetostrictive resonator coupled to each of said inductances to be mechanically driven by alternating voltages appearing in its corresponding inductance.

11. In combination, a magnetostrictively-driven electromechanical lter having driving and pickup coils coupled thereto, a separate L-section resonant LC impedance matching network coupled to each of said coils for connecting such coils to corresponding impedances, and a separate sharply resonant magnetostrictive resonator coupled to the inductances of each of said networks to be mechanically driven by alternating voltages appearing in its corresponding inductance.

12. In combination, a magnetostrictively-driven electromechanical lter having driving and pickup coils coupled thereto, a separate L-section resonant LC impedance matching network coupled to each of said coils for connecting such coils to corresponding electron discharge tube stages, and a separate sharply resonant magnetostrictive resonator coupled to the inductance of each of said networks to be mechanically driven by alternating voltages appearing in its corresponding inductance.

LESLIE L. BURNS, JR. WALTER VAN B. ROBERTS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

